As the almost only means for measuring a phenomenon that occurs in an extremely short time period of the order of picosecond (ps) or femtosecond (fs), the pump probe method using an ultrashort pulse laser beams is known. The pump probe method is available in various embodiments. All the embodiments measure probe signals as a function of delay time, which is the duration from the excitation of a sample by pump light to the detection of the state of the sample by probe light. Ultrafast response of the sample to photoexcitation can thus be measured while achieving high time resolution of the order of femtosecond.
Since the intensity of signals obtained by this measurement method is low in general, modulation measurement is performed to pick up desired signals from noise. The modulation of the pump light intensity was the most common modulation measurement method. This method has achieved good results in researches in which time-resolved measurement is conducted to measure the reflectance of probe light.
Depending on measurement environment, however, modulation of high-intensity pump light causes the sample temperature to increase and decrease repetitively, which may adversely affect the measurement. This problem has been a concern in a series of researches to create a so-called time-resolved scanning probe microscope capable of achieving ultimate resolution in both time and space regions by combining the pump probe method and a scanning probe microscope.
On the other hand, as a method that does not modulate light intensity, the delay time modulation method is known. The conventional delay time modulation method changes optical path length by physically moving the position of a mirror placed on the pump light path to adjust delay time. Consequently, with the delay time modulation method, the limited capacity of a driving mechanism for physically transferring the mirror restricts the transfer distance of the mirror, causing delay time setting range to be limited to 1 ns or shorter, and delay time modulation range to 100 ps or shorter. In addition, since modulation cannot be realized with a large-amplitude and high-frequency, modulation frequency is limited to 20 Hz or lower, for example. Thus, the measurement of a phenomenon having relaxation time of several hundred ps or longer, for example, cannot be carried out. Furthermore, the mechanical transfer of the mirror position vibrates an optical bench, causing the optical axis to deviate and eventually decreasing measurement accuracy.
Moreover, since the signal intensity is proportional to the modulation amplitude in this delay time modulation method, modulation amplitude must be increased to measure small and gradual relaxation processes.
However, increasing modulation amplitude decreases modulation frequency and thus increases noise amplitude. In addition, since output signals are the average value over the entire modulation amplitude, time resolution decreases. Furthermore, since differentiated signals are measured with this delay time modulation method in principle, the absolute value of signals cannot be found, which makes physical interpretation difficult.
Meanwhile, taking advantage of the feature that it does not cause increase and decrease of the temperature of samples and the measurement system, this conventional delay time modulation method was combined with a scanning probe microscope into a delay-time-modulated scanning probe microscope, which has accomplished certain results.
On the other hand, a new delay time modulation method was designed recently. It has two laser oscillators oscillating at repetition frequencies slightly differing from each other and are used for each laser as a pump light and a probe light. In this case, the delay time is scanned from zero to the repetition period at high speed at the modulation frequency equivalent to the difference between the two repetition frequencies. By recording probe signals in synchronization with the frequency equivalent to the difference between the above repetitive frequencies and averaging the obtained values, the measurement can be performed easily even in a large delay time region without mechanically changing the mirror position, and the absolute signal values can thus be measured. Furthermore, this method is superior to the previously mentioned methods in that modulation frequency can be increased.
Nonpatent literature 1: A. Bartels, Appl. Phys. Lett. 88, 041117 (2006)